Metal mold for centrifugal casting

ABSTRACT

A metal mold for centrifugal casting, the exterior surface of which is provided with a coating in the form of an iron oxide film or a film containing calcium as the main ingredient. Such a coating is preferably formed so as to substantially cover the entire exterior surface. If the coating is an iron oxide film, it is preferable for the film to contain triiron tetraoxide. If the coating is a film containing calcium as the main ingredient, it is preferable for the thickness thereof to be at most 0.4 mm. Furthermore, among films containing calcium as the main ingredient, it is preferable for said main ingredient to be calcium carbonate. By configuring a mold for centrifugal casting in such a manner, the exterior surface of the metal mold can be uniformly cooled, and deformation of the mold can be suppressed.

TECHNICAL FIELD

The present invention relates to a metal mold for centrifugal casting.

BACKGROUND ART

There is a known centrifugal casting method in which a cylindrical mold is rotated at high speed around its axis and an injected molten metal is stuck to an inner wall of the metal mold by centrifugal force to manufacture a tubular member as a hollow casting article. In the centrifugal casting method, in order to easily extract the molded casting article from the mold and to impart a shape to an exterior surface of the hollow casting article, in some cases, a method of applying a coating material in the mold may be adopted. In this case, in order to facilitate the application of the coating material and to dry the applied coating material, it is desirable to cool the mold to an appropriate temperature in advance when applying the coating material. For this reason, after the casting article molded from the mold in the preceding casting cycle is extracted, the exterior surface of the mold is cooled before the coating material is applied in the next casting cycle.

As described above, there is proposed a technique of cooling the inside of the casting mold after casting the casting article in the preceding casting cycle until the cycle is shifted to the next casting cycle (for example, Patent Document 1). Patent Document 1 discloses a technique of recovering mist generated from a casting mold to prevent the deterioration of the peripheral equipment and to improve the working environment, when the cooling liquid is sprayed or scattered to the interior of the high-temperature casting mold to perform cooling.

-   Patent Document 1: Japanese Unexamined Patent Application,     Publication No. 2004-160500

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

When cooling the exterior surface of the metal mold for centrifugal casting, there was a difference in temperature drop for each part on the exterior surface of the metal mold, and the metal mold was deformed due to the temperature difference. As a result of the deformation, there was a problem in which the mold release resistance when extracting the casting article in the metal mold increases, it is necessary to use a strong and large device for extracting the casting article, and the facility of the centrifugal casting process increases. However, the technique disclosed in Patent Document 1 is focused on recovery of mist generated when the casting mold is cooled, and there is no separate viewpoint on the technical problem such as deformation that occurs when the metal mold for centrifugal casting is cooled from its exterior surface.

Thus, an object of the present invention is to provide a metal mold for centrifugal casting capable of uniformly cooling the exterior surface of the metal mold and suppressing deformation of the metal mold.

Means for Solving the Problems

The inventors found that there is a difference in the degree of occurrence of the Leidnfrost phenomenon for each part on the surface of the metal mold as a cause of the temperature drop at the time of cooling of the metal mold for centrifugal casting being different for each part. That is, the exterior surface of the metal mold has risen to about 300° C. due to the heat of the molten metal poured into the mold. In this state, when cooling is tried, the cooling water instantaneously evaporates on the exterior surface of the metal mold and a layer of evaporated gas is generated between the exterior surface of the metal mold and the liquid to cause a Leidenfrost phenomenon which inhibits heat conduction, but the degree of occurrence of this phenomenon varies for each part on the metal mold surface. The inventors focused attention on the above points and found that in order to suppress the occurrence of the Leidenfrost phenomenon and to make the cooling effect of the metal mold uniform, a coating having a heat insulating property is applied to the surface of the metal mold to lower the interface temperature, thereby solving the aforementioned problem. Here, the following techniques are proposed.

(1) A metal mold for centrifugal casting (for example, a metal mold for centrifugal casting 10 to be described later), including a coating (for example, a coating 10 a to be described later) on an exterior surface, in which the coating is an iron oxide film or a film containing calcium as a main ingredient.

In the metal mold for centrifugal casting of the above (1), since the interface temperature of the metal mold remains within the range that does not cause the Leidenfrost phenomenon by the coating which is the iron oxide film or the film containing calcium as the main ingredient, the uniform cooling of the exterior surface of the metal mold for centrifugal casting is not inhibited by the Leidenfrost phenomenon. Therefore, deformation due to the temperature difference in the metal mold for centrifugal casting is suppressed.

(2) In the metal mold for centrifugal casting of (1), the coating may be formed to cover substantially the entire surface of the exterior surface.

In the metal mold for centrifugal casting of (2), particularly in the above-mentioned metal mold for centrifugal casting of (1), the coating is formed to cover substantially the entire surface of the exterior surface. For this reason, the cooling of the entire metal mold for centrifugal casting becomes uniform, and deformation is reliably suppressed.

(3) In the metal mold for centrifugal casting of (1) or (2), the coating may be a film containing triiron tetraoxide in the iron oxide film.

According to the metal mold for centrifugal casting of (3), the effect of suppressing the Leidenfrost phenomenon is particularly good. Therefore, the cooling of the metal mold for centrifugal casting becomes uniform, and the deformation is reliably suppressed.

(4) In the metal mold for centrifugal casting of (1) or (2), the coating may be a film containing calcium as the main ingredient and a thickness thereof may be 0.4 mm or less.

According to the above-mentioned metal mold for centrifugal casting of (4), the effect of suppressing the Leidenfrost phenomenon is particularly good. Therefore, the cooling of the metal mold for centrifugal casting becomes uniform, and the deformation is reliably suppressed.

(5) In the metal mold for centrifugal casting of (1) or (4), the coating may be a film containing calcium carbonate as a main ingredient in the film containing calcium as the main ingredient.

According to the metal mold for centrifugal casting of (5), in particular, the effect of suppressing the Leidenfrost phenomenon is particularly good. Therefore, the cooling of the metal mold for centrifugal casting becomes uniform, and the deformation is reliably suppressed.

Effects of the Invention

According to the present invention, it is possible to provide a metal mold for centrifugal casting capable of uniformly cooling the exterior surface of the metal mold and suppressing deformation of the metal mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process chart of centrifugal casting performed using a metal mold for centrifugal casting according to an embodiment of the present invention.

FIG. 2 is a view illustrating a temperature history of the metal mold for centrifugal casting in the process of FIG. 1.

FIG. 3 is a diagram illustrating the temperature dependency of an amount of bending at the time of cooling of the metal mold for centrifugal casting.

FIG. 4 is a conceptual diagram illustrating a coating applied to the metal mold for centrifugal casting according to an embodiment of the present invention.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

First, the process of centrifugal casting performed, using the metal mold for centrifugal casting of the present invention will be described, and the cooling from the exterior surface of the metal mold for centrifugal casting, which is an object of the present invention, will be described.

FIG. 1 is a process chart of centrifugal casting performed using a metal mold for centrifugal casting of the present invention. FIG. 1 illustrates a process of one cycle of the centrifugal casting. In FIG. 1, the process of one cycle is performed in the order of (a)→(b)→(c)→(d)→(e)→(f), and the one cycle is cyclically repeated, and a casting article is made every time. In the case of this example, the casting article is a tubular body, and a casting article, which is one tubular body, is cut into a plurality of pieces with a predetermined length and processed into a cylinder sleeve of an engine, respectively.

In the aforementioned process, (a) is a metal mold cleaning process, (b) is a mold temperature adjusting process, (c) is a coating mold application process, (d) is a pouring process, (e) is a cure process, and (f) is an article extracting process. In FIG. 1, a cylindrical metal mold for centrifugal casting 10 is installed such that its axis extends in a substantially horizontal. The metal mold for centrifugal casting 10 is supported by two pairs of rollers so as to be rotatable around the axis. In FIG. 1, the two sets of rollers are a pair of rollers 21 on a left side and a pair of rollers 22 on a right side. The rollers 21 and rollers 22 are rotationally driven by a power source such as a motor (not illustrated), and the metal mold for centrifugal casting 10 rotates around its axis in accordance with the rotation. An annular frame body 11 with an opening (not illustrated) provided at a center portion is fitted to one end side (the left side in FIG. 1) of the metal mold for centrifugal casting 10. An end cap 12 is removably fitted to the other end side (the right side in FIG. 1) of the metal mold for centrifugal casting 10. The metal mold for centrifugal casting 10 continues to rotate in processes other than the article extracting process of (f). Further, the metal mold for centrifugal casting 10 has a specific feature in which a specific coating is applied to its exterior surface, which will be described in detail later.

Next, the process of one cycle from (a) to (f) will be sequentially explained. In the metal mold cleaning process of (a), in the previous cycle, the workpiece due to the casting mold and the casting article is extracted, and the inner circumferential surface of the high-temperature metal mold for centrifugal casting 10 is cleaned by a brush 31 attached to a tip side of a cleaning mechanism in which a rod 30 is partially illustrated. The rod 30, that is, brush 31 reciprocates in an axial direction of the metal mold for centrifugal casting 10, and at this time, the metal mold for centrifugal casting 10 continues to rotate around its axis, for example, at a low speed of about 180 rpm. As a result, the entire mold as the inner peripheral surface of the metal mold for centrifugal casting 10 is cleaned.

In the metal mold for centrifugal casting 10 in which the entire inner peripheral surface has been cleaned in the metal mold cleaning process of (a), a coating material is applied to a mold which is the inner peripheral surface thereof to form a coating mold. The coating mold is formed for the purpose of easily extracting the molded casting article from the mold, but in the case of this example, there is also a purpose of forming a large number of fine protrusions or spine body (spiny) on the exterior surface of the hollow casting article. The spiny is transferred to an exterior surface of a cylinder sleeve processed from a hollow casting article cast using the above-mentioned coating mold. When such a cylinder sleeve is disposed and cast into the engine block, the cylinder sleeves are firmly held at their proper position by the spiny of the exterior surface.

In order to facilitate application of the coating material and efficiently dry the applied coating material, it is necessary to cool the metal mold for centrifugal casting 10 in advance to an appropriate temperature. Therefore, in a mold temperature regulating process of (b) after the metal mold cleaning process of (a) described above, the cooling water is sprayed from the spray nozzle 41 onto the exterior surface of the metal mold for centrifugal casting 10 to cool the metal mold for centrifugal casting 10. That is, the metal mold for centrifugal casting 10 is cooled by the vaporization heat of the cooling water when the cooling water is sprayed onto the exterior surface of the high-temperature metal mold for centrifugal casting 10. The spraying of the cooling water is performed, while rotating the metal mold for centrifugal casting 10 at a low speed of, for example, about 180 rpm in a state in which an end cap 12 is fitted to the metal mold for centrifugal casting 10.

However, in the related art, when carrying out the cooling, as described above, there is a difference in temperature drop on the exterior surface of the metal mold for centrifugal casting 10, and the metal mold is deformed due to the temperature difference. If the metal mold for centrifugal casting 10 is deformed, there is a technical problem in which the mold releasing resistance when extracting the casting article increases and it is necessary to use a powerful and large device as the workpiece extraction device. Means for solving this technical problem is a specific coating applied to the exterior surface of the metal mold for centrifugal casting 10, but this will be described below in one cycle process from (a) to (f).

In the mold temperature adjusting process of (b), the coating material 60 is applied to the entire mold which is the inner circumferential surface in the following coating mold application process of (c), on the metal mold for centrifugal casting 10 cooled to an appropriate temperature by the spraying of the cooling water from the spray nozzle 41. The application of the coating material 60 in the coating mold application process of (c) is carried out, by spraying the coating material 60 onto the entire mold which is the inner circumferential surface of the metal mold for centrifugal casting 10 from the nozzle 52 of the tip of the hollow shaft 51 which extends from the coating material supply device (not illustrated) through an opening of the end cap 12 inside the metal mold for centrifugal casting 10. The hollow shaft 51 and the nozzle 52 spray the coating material 60, while moving in the longitudinal direction in the mold for centrifugal casting 10.

On the other hand, since the metal mold for centrifugal casting 10 itself continues to rotate at a moderate speed of, for example, about 1500 rpm, the coating material 60 is gradually applied to the entire mold which is the inner peripheral surface, and the coating mold 61 is formed. The temperature of the coating mold 61 at the time of applying the coating material 60 is, for example, about 150° C. to 350° C.

In the coating mold application process of (c), when the coating mold 61 is formed on the entire inner circumferential surface of the metal mold for centrifugal casting 10, then the molten metal 70 is poured in the pouring process of (d). The molten metal 70 is prepared in a melting furnace (not illustrated), a fixed quantity thereof is stored in the pot 71, and is poured from the pot 71 into the metal mold for centrifugal casting 10 through the pouring pipe 73 of the trough 72 and into the coating mold 61 accordingly. At this time, since the metal mold for centrifugal casting 10 continues to rotate at a high speed of, for example, about 2000 rpm, a centrifugal force acts on the molten metal 70, and for example, in the curing process of (e) after about 90 seconds after elapse of the pouring, a uniform casting article 74 is formed on the inner surface side of the coating mold 61.

Next, in the article extracting process of (f), the rotation of the metal mold for centrifugal casting 10 is stopped and the end cap 12 is removed. In this state, the workpiece with the coating mold 61 and the casting article 74 integrated is extracted using the workpiece extraction device 80. The mold releasing resistance in the extraction is, for example, about 5.0 kN to 9.0 kN. When the coating mold 61 is removed from the extracted workpiece, a casting article 74 having a spiny transferred to its exterior surface is obtained. In this case, the weight of the casting article 74 is, for example, about 10 kg to 35 kg.

When the article extracting process of (f) is completed and one cycle of the process is completed, the next one cycle starts again from the metal mold cleaning process of (a).

FIG. 2 is a diagram illustrating a history of the temperature of the metal mold for centrifugal casting in the process of FIG. 1. In FIG. 2, a horizontal axis represents the transition (that is, the transition of time) of the one cycle of processes (a)→(b)→(c)→(d)→(e)→(f) aforementioned in FIG. 1, and a vertical axis represents the temperature (° C.). An example of an elapsed time (an approximate time zone in which the observation was made in this example) according to the respective processes (a), (b), (c), (d), (e) and (f) in FIG. 2 and the temperature of the metal mold for centrifugal casting is more specifically illustrated in the following table.

TABLE 1 Process Elapsed time sec Mold temperature ° C. (a) 15-30 275-350 (b) 55-99 150-250 (c) 119-175 130-250 (d) 124-187 130-270 (e) 177-249 275-350 (f) 186-261 275-350

That is, the temperature of the metal mold for centrifugal casting in the process of FIG. 1 transits as qualitatively illustrated in FIG. 2 with the progress of the process, and more specifically, the temperature has the values as illustrated in the above table. Therefore, also considering the temperature dependency of the amount of bending at the time of cooling the metal mold for centrifugal casting as described below, the temperature range in the above table is considered. FIG. 3 is a diagram illustrating the temperature dependency of the amount of bending at the time of cooling the metal mold for centrifugal casting. The characteristics of FIG. 3 are those found by experiments performed by the inventors and others, while repeatedly considering them. In FIG. 3, Tk in the horizontal axis represents the temperature [° C.] of the metal mold for centrifugal casting 10, and Ti represents the temperature [° C.] of the interface. Here, the interface is the boundary surface between the metal mold for centrifugal casting 10 and the cooling water. In the case of the present embodiment, the exterior surface of the specific coating applied to the exterior surface of the metal mold for centrifugal casting 10 is the interface. Ti is written to correspond to each value of Tk, Ti is lower than Tk as written, and a temperature gradient corresponding to the difference between Tk and Ti is generated in the thickness direction of the corresponding coating. Also, the vertical axis is the amount of bending [mm] of the metal mold for centrifugal casting.

As understood from FIG. 3, on the basis of a boundary in which the temperature Tk of the metal mold for centrifugal casting 10 is 270 [° C.] and the temperature Ti of the corresponding interface is 245 [° C.], in a region Z1 under the boundary, the amount of bending remains in a problem-free range such as 0.05 [mm] to 0.15 [mm]. In contrast, a region Z2 above the boundary, the amount of bending increases discontinuously, such as 0.30 [mm] to 0.65 [mm]. Accordingly, in the region Z2, with reference to FIG. 1, the mold releasing resistance in the article extracting process of (f) described above increases from the metal mold for centrifugal casting 10, causing problems in specification required for the workpiece extraction device 80, the quality of the casting article and the like. That is, if the metal mold for centrifugal casting 10 is eccentric due to bending, defects of uneven thickness of the casting article occur, and the spiny also varies.

As described above, at the boundary value in which the temperature Tk of the metal mold for centrifugal casting 10 of 270 [° C.] and the temperature Ti of the corresponding interface is 245° C., it is presumed that a Leidenfrost phenomenon actively occurs in the temperature region higher than the boundary value. That is, in the high-temperature region, due to the Leidenfrost phenomenon, on the exterior surface of the metal mold for centrifugal casting 10, there is a difference in cooling effect depending on the parts, and as a result of non-uniform temperature distribution, it is presumed that an undesired bending occurs in the metal mold for centrifugal casting 10.

Therefore, in order to suppress the Leidenfrost phenomenon and to efficiently cool the metal mold for centrifugal casting 10, it is effective to suppress the occurrence of bending by keeping the temperature Ti of the interface so as to be equal to or less than the boundary value in which the temperature Tk of the metal mold for centrifugal casting 10 is 270 [° C.] and the temperature Ti of the interface corresponding thereto is 245 [° C.].

In order to suppress the occurrence of the Leidenfrost phenomenon and make the temperature Ti of the interface below the boundary value of 245 [° C.] to make the cooling effect of the metal mold uniform, the inventors and the like solve the aforementioned problems, by applying a heat insulating coating on the exterior surface of the metal mold for centrifugal casting 10, and by lowering the interface temperature.

FIG. 4 is a conceptual diagram illustrating a coating applied to a metal mold for centrifugal casting according to an embodiment of the present invention. FIG. 4 particularly illustrates the metal mold for centrifugal casting 10 itself and a coating 10 a applied to the exterior surface in terms of its cross-sectional view, in the metal mold for centrifugal casting 10 described above with reference to FIG. 1. In the present embodiment, the coating 10 a is a specific one as described below, and by applying such a coating 10 a, the aforementioned technical problem is solved.

Here, the temperature Ti of the interface is calculated by the following relational formula (1).

$\begin{matrix} {\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \mspace{625mu}} & \; \\ {{Ti} = {\frac{T_{k} - T_{w}}{1 + \sqrt{\frac{\left( {\rho \; {Cp}\; \lambda} \right)_{w}}{\left( {\rho \; {Cp}\; \lambda} \right)_{k}}}} + T_{w}}} & (1) \end{matrix}$

In this relational formula (1), Tk is the temperature of the metal mold for centrifugal casting 10, Tw is the temperature of the cooling water, ρ is the specific gravity, Cp is the specific heat, and λ is the thermal conductivity. Further, (ρ·Cp·λ) is the physical property value, and in the relational formula, (ρ·Cp·λ)_(k) is the physical property value of the coating 10 a of the metal mold for centrifugal casting 10, and (ρ·Cp·λ)_(w) is the physical property value of the cooling water.

As described above, the Leidenfrost phenomenon is hard to occur if the temperature Ti of interface is equal to or lower than the boundary value of 245 [° C.]. Therefore, a material exhibiting the physical property value satisfying the condition that the interface temperature Ti is 245 [° C.] or less is suitable for the coating 10 a on the exterior surface of the metal mold for centrifugal casting 10.

From the above relational formula, in the coating 10 a having the thermal conductivity λ of 2.0 W/m·K or less satisfies the condition that the temperature Ti of interface is 245° C. or less. Therefore, by applying such a coating 10 a to the exterior surface of the metal mold for centrifugal casting 10, the occurrence of the Leidenfrost phenomenon is suppressed, and it is found that the undesirable bending of the metal mold for centrifugal casting 10 due to unevenness of cooling in the cooling process is prevented.

Then, the inventors and the like repeated various experiments to satisfy the above-mentioned conditions, and specified a film suitable for the coating 10 a to be applied to the exterior surface of the metal mold for centrifugal casting 10. That is, a black rust called “black dye”, that is, a triiron tetraoxide (Fe₃O₄) film is an example. In this case, since iron hydroxide (FeO(OH)) is usually generated at the same time as triiron tetraoxide (Fe₃O₄), iron hydroxide is also considered to be suitable as a film forming the coating 10 a. That is, the iron oxide film is suitable for coating to be applied to the exterior surface of the metal mold for centrifugal casting. Further, it was confirmed that a film containing calcium carbonate as a main ingredient formed by reaction of lime and mineral (Ca) in water was suitable for the coating 10 a. That is, a film containing calcium as a main ingredient is suitable for coating on the exterior surface of the metal mold for centrifugal casting.

Particularly, in the case of forming the coating 10 a on the exterior surface of the metal mold for centrifugal casting 10 by a film containing calcium carbonate (CaCO₃) or calcium carbonate as a main ingredient, when the thickness of the coating 10 a is 0.4 [mm] or less, it is possible to effectively suppress the Leidenfrost phenomenon and to efficiently cool the metal mold for centrifugal casting 10.

The thickness L of the coating 10 a as described above is calculated as a value satisfying the following relational formula (2) under the conditions illustrated in the following table. That is, the following relational formula (2) was applied for calculation:

$\begin{matrix} {\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack \mspace{625mu}} & \; \\ {Q_{J} = {\frac{\lambda}{L}\left( {\theta_{1} - \theta_{2}} \right)\mspace{14mu} {At}\mspace{14mu} J}} & (2) \end{matrix}$

In the relational formula (2), L is the thickness of the coating 10 a, Q_(J) is the amount of heat that requires heat radiation, λ is the thermal conductivity (in the case of this example, thermal conductivity of coating 10 a containing calcium carbonate (CaCO₃) or calcium carbonate as a main ingredient), θ₁ is the temperature of the coating 10 a on the side being in contact with the metal mold for centrifugal casting 10, θ₂ is the temperature of the exterior surface to which the cooling water is applied in the coating 10 a, A is an area of the exterior surface of the coating 10 a, and t is an elapsed time. In this case, if the elapsed time t is set to be too long, since the cycle time of casting extends and the productivity is lowered, 30 seconds is set as an upper limit.

When calculating the thickness L of the coating 10 a by applying the above relational formula (2), the set conditions are as illustrated in the following table.

TABLE 2 Q λ Amount of Heat A t Lmm heat conduction θ₁ θ₂ Area Time 0.40715 6000000 0.6 280 180 1.357168 30 0.271434 6000000 0.6 280 180 1.357168 20 0.203575 6000000 0.6 280 180 1.357168 15

The operational effects of the metal mold for centrifugal casting 10 of the present embodiment described above are summarized. (1) In the metal mold for centrifugal casting 10, since the interface temperature of the metal mold remains within the range that does not cause the Leidenfrost phenomenon by the coating which is the iron oxide film or the film containing calcium as the main ingredient, the uniform cooling of the exterior surface of the metal mold for centrifugal casting 10 is not inhibited by the Leidenfrost phenomenon.

Therefore, deformation due to the temperature difference in the metal mold for centrifugal casting 10 is suppressed.

(2) The coating 10 a is formed to cover substantially the entire surface of the exterior surface of the metal mold for centrifugal casting 10. Therefore, the cooling of the entire metal mold for centrifugal casting 10 becomes uniform, and deformation is reliably suppressed.

(3) Since the coating 10 a is a film containing triiron tetraoxide (Fe₃O₄) in the iron oxide film, the effect of suppressing the Leidenfrost phenomenon is good. Therefore, the cooling of the metal mold for centrifugal casting 10 becomes uniform, and deformation is reliably suppressed.

(4) The coating 10 a is a film containing calcium as a main ingredient, and its thickness is 0.4 mm or less. Therefore, the effect of suppressing the Leidenfrost phenomenon is good. Accordingly, the cooling of the metal mold for centrifugal casting becomes uniform, and the deformation is reliably suppressed. (5) The coating 10 a is a film containing calcium carbonate (CaCO₃) as a main ingredient in the film containing calcium as a main ingredient. Therefore, the effect of suppressing the Leidenfrost phenomenon is good. Accordingly, the cooling of the metal mold for centrifugal casting becomes uniform, and the deformation is reliably suppressed.

Further, the present invention can be implemented with various modifications other than the above-described aspects. The above example was a metal mold for centrifugal casting applied to the manufacturing process of the cylinder sleeve (cylinder liner) of the engine. However, the present invention is not limited to this aspect, but can be applied to various metal molds for centrifugal casting. When it is applied at the time of manufacturing relatively long tubular body or the like, since the bending at the time of cooling is small, the extraction of the casting article is easy, which is a great advantage from the viewpoint of production efficiency and quality control. Here, the advantages in terms of quality control specifically mean that the bending at the time of cooling of the metal mold for centrifugal casting is reduced, defects of uneven thickness of the casting article are hard to occur, and variations in formation of the spiny are also hard to occur.

EXPLANATION OF REFERENCE NUMERALS

-   10 METAL MOLD FOR CENTRIFUGAL CASTING -   10 a COATING -   41 SPRAY NOZZLE -   60 COATING MATERIAL -   61 COATING MOLD -   70 MOLTEN METAL -   74 CASTING ARTICLE 

1. A metal mold for centrifugal casting, comprising: a coating on an exterior surface, wherein the coating is an iron oxide film or a film containing calcium as a main ingredient.
 2. The metal mold for centrifugal casting according to claim 1, wherein the coating is formed to cover substantially the entire surface of the exterior surface.
 3. The metal mold for centrifugal casting according to claim 1, wherein the coating is a film containing triiron tetraoxide in the iron oxide film.
 4. The metal mold for centrifugal casting according to claim 1, wherein the coating is a film containing calcium as the main ingredient and a thickness thereof is 0.4 mm or less.
 5. The metal mold for centrifugal casting according to claim 1, wherein the coating is a film containing calcium carbonate as a main ingredient in the film containing calcium as the main ingredient.
 6. The metal mold for centrifugal casting according to claim 2, wherein the coating is a film containing triiron tetraoxide in the iron oxide film.
 7. The metal mold for centrifugal casting according to claim 2, wherein the coating is a film containing calcium as the main ingredient and a thickness thereof is 0.4 mm or less.
 8. The metal mold for centrifugal casting according to claim 2, wherein the coating is a film containing calcium carbonate as a main ingredient in the film containing calcium as the main ingredient.
 9. The metal mold for centrifugal casting according to claim 3, wherein the coating is a film containing calcium carbonate as a main ingredient in the film containing calcium as the main ingredient.
 10. The metal mold for centrifugal casting according to claim 4, wherein the coating is a film containing calcium carbonate as a main ingredient in the film containing calcium as the main ingredient. 